In commercial electronic product packaging, paper-base-phenolic resin boards predominantly are used in printed circuit board electronics. Circuitry, commonly in the form of circuit traces, is printed on one side of the board. Through holes are die punched through the board with no plating. Electronic components, such as connectors, are mounted on the "bare" side of the board; i.e., the side opposite the printed circuitry. Leads, usually in the form of terminal pins, project out of the connectors through the holes in the board to the circuitry side thereof for dip or wave soldering to the circuit traces. These solder connections or joints are very small and fragile.
One of the major problems with such electronics packaging is that the solder joints become damaged from cracking when the board is exposed to thermal cycling as a result of differences in thermal expansion between the board and the electronic components.
Among the components mounted to printed circuit boards, connectors are of the most prone to cause cracking primarily because of their bulky size. Since there is a finite gap between a terminal pin and the wall of a hole in the printed circuit board, and because of the high stiffness of the terminal pin, the mismatch between the thermal expansions of the paper based board and the connector result in a rocking motion with the solder joint as a pivot. Deformation is concentrated primarily at the solder joint, and the joint is extremely vulnerable to thermal cycling whereupon cracking results after repeated motion.
Cracking of solder joints is a major cause of failure in electronic packages. With popular, widely used surface mount methods, cracking of solder joints is even more disturbing because it results in complete separation of the terminal pin from the board.
One solution to the cracking problem is to strengthen the solder joint by improving the strength of the solder material. Improving the mechanical strength of the solder by alloying different constituents, however, inevitably results in a higher melting temperature than the usual eutectic solder, or increased brittleness that can make the joint even more prone to cracking. Another solution is to minimize the stress caused by the thermal mismatch between the board and the components by selecting materials with coefficients of thermal expansion close to each other. However, the use of paper-base-phenolic resin boards and plastic connector components and the conductive terminal pins has been arrived at, over time, for many considerations which do not lead to change. Still another approach would be to design the leads or terminal pins to be sufficiently compliant to absorb the expansion mismatches without accumulating stress. However, this tends to make the terminal pins too fragile during manufacturing processes and later handling. All of these considerations must be viewed in light of the fact that the connectors or other components and the board are interactive; i.e., the forces resulting from a thermal mismatch may be allocated to the board itself rather than the solder joints alone.
This invention is directed to solving the above problems and dilemmas by providing a new and improved terminal pin or lead which is designed to absorb thermal expansion mismatches in the electronic circuitry packaging without accumulating stresses in the lead.